Patent Document No. 1 discloses a conventional barrier unit. FIG. 9 is a cross-sectional view illustrating a barrier unit 101 as disclosed in Patent Document No. 1.
At the center of a frontend plate portion 101A, there is a window 101a that makes an optical path with respect to a lens 109. A set of barrier blades 104, which is arranged in front of the frontend plate portion 101A, opens and closes the window 101a. When the barrier blades 104 are open, a shutter opens and closes on a shutter plate 110, thereby getting shooting done.
In this barrier unit 101, when the barrier blades 104 are fully opened or closed, a barrier blade drive ring 102 protrudes toward the direction from which light is coming, thereby pressing the barrier blades 104 against a barrier protective cap 108. As a result, sand or any other foreign matter will not easily enter the gap between the barrier blades 104 and the barrier protective cap 108.
Hereinafter, a typical conventional barrier unit will be described with reference to FIGS. 10 through 14.
FIG. 10 is a plan view illustrating a situation where first and second blades 2 and 3 are opened, while FIG. 11 is a plan view illustrating a situation where the first and second blades 2 and 3 are closed. In FIGS. 10 and 11, the first and second blades 2 and 3 are shown in a simplified form.
The first blade 2 has a pivot hole 15 and an elongate hole 17, while the second blade 3 has a pivot hole 16 and an elongate hole 18. The first and second blades 2 and 3 are driven at the elongate holes 17 and 18 so as to pivot around the pivot holes 15 and 16, respectively. When opening or closing, the first and second blades 2 and 3 are guided on blade guide rail portions 1a of the cap 1 and on blade guide rail portions on a base as will be described later. When the first and second blades 2 and 3 are closed, the first and second edges 2a and 3a of the first and second blades 2a and 3a are preferably in contact with each other without overlapping with each other.
Hereinafter, it will be described, with reference to cross-sectional views as viewed on the plane C-C shown in FIG. 11, what may happen when the first and second blades 2 and 3 are closed.
FIGS. 12 to 14 are cross-sectional views illustrating, as viewed on the plane C-C, the relative positions of the first and second blades 2 and 3 in mutually different states.
Specifically, FIG. 12 illustrates a situation where the first and second blades 2 and 3 with ideal planar surfaces are closed right in their intended positions. In FIG. 12, when opening or closing, the first and second blades 2 and 3 are guided along the blade guide rail portions 1a on the cap 1 and along the blade guide rail portions 4a on the base 4. To open and close these blades just as intended, some clearance d is left between the first and second blades 2, 3 and the blade guide rail portions 1a and 4a. 
In the example illustrated in FIG. 12, the first and second blades 2 and 3 are substantially level with each other and closed without imperfectly fitting with each other vertically in the drawing. As used herein, “vertically” means perpendicularly to the cap 1 and the base 4.
On the other hand, FIG. 13 illustrates a situation where the first and second blades 2 and 3 with ideal planar surfaces are closed but have failed to perfectly fit with each other vertically. And FIG. 14 illustrates a situation where the first and second blades 2 and 3 with a low degree of planarity are closed. As used herein, the “low degree of planarity” refers to non-planar first and second blades 2 and 3, which may be wavy or curved as shown in FIG. 14, for example.                Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 9-61887        